[Qemu-devel] [PATCH 2/2] virtio-scsi-dataplane: notify guest as batch

[Ming Lei <ming.lei@canonical.com>]

It isn't necessery to notify guest each time when one request
is completed, and it should be enough to just notify one time
for each running of virtio_scsi_iothread_handle_cmd().

This patch supresses about 30K/sec write on eventfd.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 hw/scsi/virtio-scsi-dataplane.c |    4 +++-
 hw/scsi/virtio-scsi.c           |   22 ++++++++++++++++++++++
 include/hw/virtio/virtio-scsi.h |    4 ++++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index 8a7cd9f..1fb83de 100644
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -62,6 +62,7 @@ static VirtIOSCSIVring *virtio_scsi_vring_init(VirtIOSCSI *s,
     aio_set_event_notifier(s->ctx, &r->host_notifier, handler);
 
     r->parent = s;
+    r->qid = n;
 
     if (!vring_setup(&r->vring, VIRTIO_DEVICE(s), n)) {
         fprintf(stderr, "virtio-scsi: VRing setup failed\n");
@@ -95,7 +96,8 @@ void virtio_scsi_vring_push_notify(VirtIOSCSIReq *req)
 {
     vring_push(&req->vring->vring, &req->elem,
                req->qsgl.size + req->resp_iov.size);
-    event_notifier_set(&req->vring->guest_notifier);
+    req->dev->pending_guest_notify |= 1 << (req->vring->qid - 2);
+    qemu_bh_schedule(req->dev->guest_notify_bh);
 }
 
 static void virtio_scsi_iothread_handle_ctrl(EventNotifier *notifier)
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 6e34a2c..1b9c35c 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -86,6 +86,21 @@ static void virtio_scsi_complete_req(VirtIOSCSIReq *req)
     virtio_scsi_free_req(req);
 }
 
+static void notify_guest_bh(void *opaque)
+{
+    VirtIOSCSI *s = opaque;
+    unsigned int qid;
+    uint64_t pending = s->pending_guest_notify;
+
+    s->pending_guest_notify = 0;
+
+    while ((qid = ffsl(pending))) {
+        qid--;
+        event_notifier_set(&s->cmd_vrings[qid]->guest_notifier);
+        pending &= ~(1 << qid);
+    }
+}
+
 static void virtio_scsi_bad_req(void)
 {
     error_report("wrong size for virtio-scsi headers");
@@ -825,6 +840,10 @@ void virtio_scsi_common_realize(DeviceState *dev, Error **errp,
 
     if (s->conf.iothread) {
         virtio_scsi_set_iothread(VIRTIO_SCSI(s), s->conf.iothread);
+        VIRTIO_SCSI(s)->pending_guest_notify = 0;
+        VIRTIO_SCSI(s)->guest_notify_bh = aio_bh_new(VIRTIO_SCSI(s)->ctx,
+                                                     notify_guest_bh,
+                                                     VIRTIO_SCSI(s));
     }
 }
 
@@ -902,6 +921,9 @@ void virtio_scsi_common_unrealize(DeviceState *dev, Error **errp)
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
     VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
 
+    if (vs->conf.iothread) {
+        qemu_bh_delete(VIRTIO_SCSI(vs)->guest_notify_bh);
+    }
     g_free(vs->cmd_vqs);
     virtio_cleanup(vdev);
 }
diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index 9e1a49c..5e6c57e 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -163,6 +163,7 @@ typedef struct {
     Vring vring;
     EventNotifier host_notifier;
     EventNotifier guest_notifier;
+    uint32_t qid;
 } VirtIOSCSIVring;
 
 typedef struct VirtIOSCSICommon {
@@ -198,6 +199,9 @@ typedef struct VirtIOSCSI {
     bool dataplane_fenced;
     Error *blocker;
     Notifier migration_state_notifier;
+
+    QEMUBH *guest_notify_bh;
+    uint64_t pending_guest_notify;
 } VirtIOSCSI;
 
 typedef struct VirtIOSCSIReq {
-- 
1.7.9.5